Astrometry satellite

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An astrometry satellite is an artificial terrestrial satellite that performs astrometry tasks in space - free from disturbing terrestrial influences .

Introduction: Observatories and the Earth's atmosphere

Much larger observatories and telescopes can be built on earth than for operation in space probes , but their qualities are usually not fully usable. The reason is mainly the earth's atmosphere , which

These disadvantages of ground-based telescopes can be partially reduced by adaptive optics and the like - but with great effort . While direction measurement in space even with much smaller instruments outperforms those in large observatories, the large apertures of earth-based telescopes are more important for measurements on weak objects . This means that space telescopes are more profitable for astrometry than for astrophysics .

Astrometry, frame of reference and satellites

For centuries, optical observations - in today's parlance astrometric observations - were the only measurements available for astronomy . Before the space age, a variety of astronomical instruments were available to define a fixed frame of reference and to study the rotation of the earth .

Earthbound Astrometry and Satellite Geodesy

In the last few decades astonishingly precise instruments have been developed and also automated for this purpose : the electronic meridian circle , the zenith telescope including the further development of the photographic zenith telescope (PZT) and the automated astrolabe of the Danjon type . They were v. a. used by those observatories that contributed to the IPMS (International Polar Motion Service). While the measurements 100 years ago approached 0.1 "at best, these measuring devices provide the geographical latitude of a station down to 0.01" (10  mas or the equivalent of 0.3 m) per night.

Nevertheless, comparable accuracies were already achieved by satellite geodesy in its second decade (around 1975 ) - admittedly not optically, but based on microwaves and EDM .
Since around 1970, the direction measurement to satellites with large satellite cameras such as the BC-4 has been around 1 ", but has hardly increased beyond 0.5" since then. However, these methods of satellite and stellar triangulation have been supplemented so powerfully by GPS and other radio wave methods that the figure of the earth can now be recorded with an accuracy of a few centimeters.
This means that optical astrometry "lags" by a factor of about ten (see above, 30 cm). This discrepancy could in part be alleviated by radio interferometry and in particular VLBI , but just as precise measurements would also be necessary in the light wave range .

Terrestrial coordinates and star catalogs

Geodesy , astronomy and mathematics are mutually dependent on each other when defining a suitable reference system for precise coordinates on earth and in space . For the time systems and because of Geodynamics and internal mass displacements have come Physics and Geophysics added.

The earth's rotation provides the connection between terrestrial coordinates and those of astronomers . The earth rotates, so to speak, in a controllable time within the astronomical coordinate frame, which is defined by its equator and the ecliptic . This reference system of the star coordinates right ascension and declination is itself variable because of the precession and nutation . Their parameters and the whole model , which is related to the earth's orbit , the moon and also the other planets , could be improved noticeably in the last decade to almost 0.01 ", which is however not enough. Numerous scientists are still working on this problem; around 40 Europeans were awarded the Descartes Prize ( Memento of February 18, 2005 in the Internet Archive ) of the EU for their research project "Non-rigid earth Nutation model" .

So while the earth measurement approached the decimeter accuracy of the earth figure in the last 10-15 years (and could reach this around 2010 cm), optical astronomy is missing by almost a factor of 10. There are improvements for such steps for the fundamental quantities and the most precise measurements of as many star locations as possible and their proper movements are required. This process stagnated almost until 1990 . The AGK - star catalog from the turn of the century, although to its quasi fifth edition improved (Fundamental catalog FK5 ), could, however, the individual star aberration of the FK4 up not entirely wipe to some 0.1 ".

The Hipparcos satellite

The first astrometry satellite Hipparcos found this gap . His name comes from the ancient astronomer who discovered precession by comparing two star catalogs ; the abbreviation is made up of HIgh Precision PARallax COllecting Satellite .

The satellite was launched by ESA in 1989 to measure a network of 120,000 stars to 0.002 ", 20–50 times more precise than previously possible. It was active until June 1993 and almost entirely achieved its goal despite a major orbit error: the Hipparcos Catalog contains 118,000 stars with 0.003 "or 0.002" / year. A second instrument measured a further 1 million star locations to 0.02 " for the Tycho-1 catalog .

At that time, these two catalogs were the most modern implementation of the International Celestial Reference Frame (ICRF). The data of 300 gigabytes provided material for around 500 specialist articles as early as 1997 - the year of publication. In 2000, the Tycho-2 catalog appeared with around 2.5 million objects as a new reduction of the existing data.

Hipparcos' measurement method was a profile- like electro-optical scanning of the stars, which are then networked to form patches by adjustment . For each measurement period so were star positions calculated, and from the time spacing of the proper motions derived. The simultaneous determination of the annual parallaxes resulted in distances of the stars 10 times more accurate than before.

The Gaia Mission

The follow-up mission , Gaia , has been running since 2013 and scans the sky with much greater accuracy. In the strict sense, Gaia is not an earth satellite, but a probe which is located approx. 1.5 million km from the earth at the sun-earth- Lagrange point L 2 . The instruments deliver not only magnitudes, star locations, parallaxes and proper motion, but also radial velocities, temperatures and spectral types. Variable stars and double stars are recognized, periodic and non-periodic magnitude changes are recorded and classified. A catalog of around 500,000 quasars could be created, which serves as a frame of reference in the optical field. In addition, solar objects are recorded and with the latest publications, exoplanets are also to be recognized for many objects.

Two preliminary catalogs have already been published. In 2016 Gaia-DR1 appeared with 1.1 billion items and in 2018 Gaia-DR2 appeared with 1.7 billion items. The next Gaia EDR3 catalog is expected to come out at the end of 2020. The mission has already been extended once and another extension is likely. The final data releases are expected approximately two to three years after the mission's final end.

See also

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